As methods for producing reduced iron, there are methods that use natural gas as a reductant and methods that use coal as a reductant. Methods that use coal have drawn much attention recently since coal is less expensive than natural gas and the geographical restriction for plant locations is less severe than the methods that use natural gas.
Regarding the methods that use coal, many proposals of methods for producing reduced iron have been made in which carbon composite metal oxide agglomerates made from a powder mixture of iron ore (metal oxide raw material) and coal (carbonaceous reductant, also referred to as “carbonaceous material” hereinafter) are charged into a rotary hearth furnace, heated, and reduced (e.g., refer to Patent Documents 1 and 2).
Examples of the methods for agglomerating a powder mixture of iron ore and coal include a method for producing spherical pellets by tumbling agglomeration using a pelletizer, a method for forming cylindrical pellets by mechanical extrusion, and a method for forming briquettes by pressing with a briquetting roll.
The method for forming pellets by tumbling agglomeration requires a large quantity of water during pelletization and thus the pellets must be dried after the agglomeration. Thus, this method has a problem in that a large heat quantity is required for drying water in the pellets (e.g., refer to Patent Document 3).
The method for forming pellets by extrusion uses fluidized raw materials; thus, a larger quantity of water is necessary during pelletization than in the case of the tumbling agglomeration described above. Moreover, the pellets must be dried after the pelletization. Thus, there is a problem in that a larger heat quantity is required for drying water in the pellets compared to the tumbling agglomeration method described above.
In contrast, in the briquetting method that uses briquetting rolls, liquid binders such as molasses (including blackstrap molasses, the same applies in the description below), lignin, and the like can be used. Thus, according to this method, dry raw materials can be agglomerated without adding water. The briquettes formed thereby do not have to be dried. Thus, there is an advantage that the heat quantity required for drying water in the agglomerates can be reduced compared to the pelletization method described above (e.g., refer to Patent Document 4).
Meanwhile, mixers used in steel industry, in particular, in iron-making processes, are large-sized to increase the throughput; thus, continuous systems are often employed to reduce the equipment cost. For example, conventional raw materials such as raw materials of sinter plants are prepared by mixing a large quantity of iron ore and a small quantity of an additive having a different property, e.g., 0.5 to 3 wt % of quick lime (for example, refer to Patent Document 8). In contrast, according to a method for producing reduced iron by heating and reducing carbon composite metal oxides in a rotary hearth furnace, as much as 30% or more of coal having a different property from iron ore is blended with iron ore (e.g., refer to Patent Document 1). In general, coal has water repellency and the bulk specific gravity of pulverized coal is smaller than that of iron ore, e.g., sometimes not more than a half that of the iron ore; thus, the volume of the coal is sometimes larger than the volume of the iron ore during mixing. The inventors of the present invention have found that conventional continuous mixers sometimes fail to achieve sufficient mixing since a large quantity of a substance having a different property is mixed with iron ore.
Moreover, this method for producing reduced iron is applicable to recycling of metal oxide-containing substances, for example, by-products such as dust generated during iron-making processes. In such a case, the raw material mixing ratio is, for example, 60 wt % converter furnace dust, 25 wt % rolling scale, and 15 wt % anthracite as a carbonaceous material in the case described in Patent Document 3, and mixing is more difficult in such a case.
In particular, when continuous mixers are used, the mixing ratio may vary when the feed rates of these powder raw materials unintentionally change over time. It has been found that this leads to problems such as insufficient reduction resulting from lack of a sufficient amount of carbon in the agglomerates required for reduction reaction in rotary hearth furnaces and insufficient strength after reduction caused by excessive amounts of carbon.
The conventional continuous methods described above are described in, for example, Patent Document 5. FIG. 2 shows one example of a conventional continuous method. Particular amounts of powders respectively stored in storage bins 21 to 24 are fed to a powder conveyor 29 so as to achieve a particular ratio. Examples of continuous constant feeding devices 25 to 28 used here include belt-conveyor-type weighing feeders and loss-in-weight feeders. The powders fed from the storage bins 21 to 24 are transferred by the powder conveyor 29 and mixed and crushed by a powder crusher/mixer device 30 such as a ball mill. Then the crushed and mixed powders are sent to a pan pelletizer 32 by a crushed material conveyor 31. In the pan pelletizer 32, the powders having water contents controlled to 8 to 13 mass % are tumbled on a wok-shaped rotary pan having a diameter of 3 to 6 m to produce spherical green pellets. Also disclosed is a method for forming compacts by using a mixer equipped with a roller having a plurality of recessed molding forms inside of which the briquettes are formed. In order to maintain the strength of the compacts, binders are usually used. A solid binder is stored in a storage bin and fed by a continuous constant feeder. A liquid binder is added into the powder crusher/mixer device 30 through a liquid feed line 33. Liquids other than binders, such as water and slurry-like raw materials (slurry containing metal oxide or carbonaceous materials), are also added in the same manner as the liquid binder.
Belt-conveyor-type continuous constant feeders measure the load on the belt and the belt speed to determine the feed rate, and the amount of feed is controlled by controlling the belt speed. This measurement method is highly accurate; however, once physical properties, such as the water content and the bulk specific gravity of the powders, vary, the feed rate changes temporarily and it takes some time before the feed rate returns to a set point and stabilizes. Loss-in-weight feeders feed the powder by a screw feeder or a table feeder while constantly measuring the weight of the powder stored in a gravimetric hopper and control the feed rate on the basis of the loss in weight. Thus, highly accurate measurement can be carried out as with the belt conveyor-type feeders. However, the feed rate also temporarily changes when physical properties, such as water content and bulk specific gravity of the powders, vary. Moreover, when the powder in the scale hopper runs low, the powder is supplemented from the storage bin disposed above, and during this operation the loss in weight cannot be measured. Thus, it has been found that there is a problem in that the feed rate can temporarily change during this operation.
It has been known that when briquettes are made from a mixture prepared by charging a liquid binder, such as molasses or lignin, and powder raw materials (iron ore and coal) into a mixer simultaneously and continuously as has been practiced widely, the strength of the briquettes varies. This is attributable to the following reason. A liquid binder is highly viscous and is thus likely to be distributed unevenly in the powder mixture. The liquid binder forms lumps in portions where the amount of the liquid binder is excessive and does not exhibit a binder function in portions where the amount of liquid binder is too small. Accordingly, the briquettes including these portions exhibit lower strength than briquettes free of portions where the binder is unevenly distributed.
In order to overcome the variation in strength of briquettes, improvements have been attempted such as changing the position where the liquid binder is fed to the continuous mixer; however, when the mixing state of the powder raw materials changes, the liquid binder just flows and glides over the surface of the powder raw materials or remains in one position. Therefore, it has been extremely difficult to stably achieve a desirable dispersion state. Furthermore, it has been found that because powder raw materials and liquid binders are fed continuously, instantaneous values of the respective feed rates vary easily, the mixing ratio of the liquid binder to the powder raw materials varies easily, and variation in strength of the briquettes still occurs.
In order to overcome the variation in strength of the briquettes, Patent Documents 6 to 8 propose methods involving diluting molasses, i.e., a type of a highly viscous liquid binder, with water to decrease its viscosity before adding the molasses to powder raw materials. However, because resultant briquettes contain large quantities of water, the briquettes must be dried after the pelletization as in the pelletizing methods described above. This leads to a problem in that a large heat quantity is required for drying water in the briquettes.
Although the field of application is different, Patent Document 9 discloses a method for stably controlling the feed rate of molasses, i.e., a highly viscous liquid binder, regardless of the temperature changes. In order to quantitatively control the flow rate of molasses despite temperature changes, a recovery tank is provided in addition to a feed tank and the flow rate is adjusted by an inverter-controlled gear pump before the valve is switched from the recovery tank to the mixer. Although a highly viscous liquid binder can be fed highly accurately by this feed method, a recovery tank, a load cell, an inverter, and controllers for controlling these components are needed, which disadvantageously increases the cost and the size of the installation site.
Patent Document 10 discloses a method involving mixing plastic waste serving as a binder with powder raw materials using a batch mixer. However, this method works on the assumption that plastic waste is reused, and the technical idea thereof is to simultaneously feed powder raw materials and thermoplastic plastic waste into a batch mixer and to soften the thermoplastic plastic waste under heating and mixing so as to cause the thermoplastic plastic waste to exhibit a binder function. Thus, the technical idea is completely different from that of the present invention described below.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2004-269978    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 9-192896    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2001-348625    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 11-310832    [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2003-89823    [Patent Document 6] Japanese Unexamined Patent Application Publication No. 7-157827    [Patent Document 7] Japanese Unexamined Patent Application Publication No. 7-224330    [Patent Document 8] Japanese Unexamined Patent Application Publication No. 2007-113086    [Patent Document 9] Japanese Unexamined Patent Application Publication No. 11-83604    [Patent Document 10] Japanese Unexamined Patent Application Publication No. 2002-235122